For me, the half-volley is a very easy shot to make. It should come naturally to those who fielded short-hops in the infield in Little League, Pony League, etc. at a young age. The keys to a successful half-volley are:
(1) Get the racket back as early as possible;
(2) Keep the racket parallel to the court surface during the entire swing;
(3) Hit the ball flat;
(4) If the ball is coming fast, you may want to start your swing even before it has crossed the net;
(5) Do not block the ball; continue with a full follow-through but one that remains parallel to the plane of the court surface;
(6) Drive the ball deep; do not dink it; and
(7) Remember timing is everything; the mechanics of the stroke are simple.
Salt air, red wine, olive oil, & Wilson Steam 99s or 105s, depending on the surface.